The metal copper has very good electrical conductivity. As a result there is currently a wide interest in the effort towards developing copper as an interconnection metalization in future packages for electronic devices. The increase in circuit density of VLSI (very large scale integration) places increasingly heavy demands on the electrical conductors on chips and on packaging substrates to which the chips are electrically interconnected. An increase in circuit density corresponds to a decrease in conductor dimensions which leads to higher resistances and current densities which increase the signal losses and promote electromigration damage to the conductors on the electronic devices and packages. The problems drive the development of high conductivity metallurgies based on copper or gold. Increased circuit density requires multiple interconnection levels which requires fully planarized structures which are achieved by complicated processes.
New materials are needed to reduce capacitive parasitics through low dielectric constants and also reduce residual stress while being susceptible to planarization. These, advanced materials and processes may have chemical incompatibilities with existing electrically conducting materials which may promote corrosion on the conductors, especially if copper is used as the electrical conductor. It becomes increasingly difficult to maintain the high conductivity of copper lines as dimensions are reduced in a case where protective overlayers are added to protect against corrosion. Since the overlayers take up space reducing room allowed for high-conductivity copper.
Gold is a commonly used corrosion resistant material. Electrical conductors on semiconductor chips and packaging substrates are commonly coated with gold to avoid corrosion. However, gold is an expensive material and must be replaced with other materials which have low resistivity and good corrosion resistance if a reduction in the cost of fabrication of electronic devices and substrates is to be achieved. For example, lead frames which have a plurality of electrically conductive leads for electrically interconnecting semiconductor chip pads to packaging substrate pads are typically formed from copper having a gold coating. Moreover, state of the art test probes as described in pending U.S. patent application Ser. No. 07/583,621, Filed Sep. 14, 1990, entitled "Flexible Tape Probe", the teaching of which is incorporated herein by reference, describes the use of gold coated copper TAB lead frames For testing semiconductor devices.
Applicants have discovered that the Si and Ge materials of the present invention provide a substantially less costly substitute for gold. Gold is resistant to corrosion, gold does not form an oxide on exposure to atmospheric oxygen and gold is solder wettable. The Si and Ge materials of the present invention are substantially as corrosion resistant as gold, do not form a surface oxide and are solder wettable.
Furthermore, gold is a commonly used surface metallization on semiconductor chip or packaging substrate contact pads to which wires are electrically connected by wire bonding and ultrasonic bonding. Quite surprisingly, it has been found that the Si and Ge materials of the present invention provide a surface as suitable as gold for wire bonding.
Furthermore, gold is a commonly used surface for metallization on semiconductor chip or packaging substrate contact pads to provide a surface to which solder, in particular, lead/tin solder can be bonded. Gold is a solder wettable surface. Quite surprisingly, it has been found that the Si and Ge materials of the present invention are solder wettable.
The properties of the Si and Ge containing materials of the present invention are comparable to that of gold. These materials are useful as an intermediate layer in multilayer metal structures, such as electrically conductive pads and lines in semiconductor chips and semiconductor chip packaging substrates.
Because the materials of the present invention are resistant to oxidization and act as a diffusion barrier to oxygen, they are useful as intermediate barrier layers to prevent oxide formation.
These and other objects, features and advantages of the present invention will become apparent from the following more detailed description and the drawings and claims appended thereto.